Many work machines use differential steering. Unlike conventional wheeled steering, which changes the orientation of the wheels, causing the machine to turn, differential steering increases the rotational speed of the ground engaging traction devices on one side of the machine relative to the other side, causing the machine to turn.
In the past, differential steering has been accomplished by mechanical and hydraulic means. The physical linkages and the sizing of the hydraulic pumps and motors contained within the differential steering system determined the steering characteristics of each machine. Once installed, the mechanical-hydraulic differential steering system maintained relatively simplistic and constant steering characteristics.
Electronic controllers are becoming increasingly popular, allowing for more complex control as compared to mechanical-hydraulic control, thereby allowing for more complex steering characteristics. A typical electronic differential steer system will include a steering wheel and steering wheel sensor that determines a position of the steering wheel, an engine and engine speed sensor, a machine speed sensor, and a gear position sensor. The sensors transmit data to the electronic controller, which controls the fluid flow from an over-center, closed-loop, variable displacement hydraulic pump. A hydraulic motor is coupled with the hydraulic pump to receive the fluid flow, and increases the rotational speed of one wheel or track on one side of the machine while decreasing the rotational speed of a second wheel or track on the other side of the machine. This difference in rotational speeds between the two sides of the machine causes the machine to turn.
In addition to the differential steering system, the machine typically includes a common drive system that provides a constant rotation to the wheels or tracks on both sides of the machine. Thus, the common mode drive system plus the differential steering system allows for complete directional movement and control of the machine.
FIG. 1 is a block diagram 10 of a conventional control scheme using closed loop control, with motor speed as a feedback term. In block 12, a steering wheel position, an engine speed, a ground speed of the machine (i.e., machine speed), and a gear position are all received. A nonlinear gain map transmits a desired motor speed signal as a function of the received information. Generally, the greater degree of turn will correspond to a higher desired motor speed. In block 14, the desired motor speed signal is summed with an actual motor speed feedback signal from block 16, discussed below, producing a motor speed error signal equal to the difference between the desired and actual motor speeds.
The motor speed error signal is input to a Proportional and Integral and Derivative (PID) controller in block 18 and processed by ways known to those skilled in the art, and transmits a scaled motor speed error signal. In block 20, the scaled motor speed error signal is processed by an electronic controller, and a pump command is transmitted as a function of the scaled motor speed error signal. In block 22, a steering pump receives the pump command, causing fluid flow as a function of the pump command. In block 24, a hydraulic steering motor receives the fluid flow, and produces a differential track speed by ways known to those skilled in the art as a function of the fluid flow. The steering motor speed is also fed back in block 16 to be summed in block 14.
This technique, however, has several heretofore-unrecognized disadvantages. First, if the feedback term from block 16 is lost, such as by a motor speed sensor failure, the technique described above will not function properly. In this instance (assuming no output from the motor speed feedback block 16), the motor speed error signal will equal the desired motor speed. When this motor speed error signal is input to the PID controller in block 18, the PID controller will amplify the motor speed error signal. Because the feedback term is zero, the motor speed error signal will be a positive value, indicating that the actual motor speed is less than the desired motor speed. The scaled motor speed error signal will cause the electronic PID controller to increase the fluid flow from the steering pump. Eventually, as the error signal continues to equal the desired motor speed signal, the output from the PID controller will saturate at its maximum, causing the pump to produce its maximum fluid flow, regardless of the steering wheel input and other inputs in block 12. In other words, this technique cannot run open-loop.
Second, this technique controls the error on the motor speed of the differential steering system, i.e., the input to the PID controller (block 18) is a motor speed. The device controlled by the controller (block 20), however, is the steering pump. Because the control action is based on the motor speed, while the controlled parameter is the displacement of the steering pump, open-loop control cannot be done directly, if at all.